Fruits are regarded as a highly desirable food, both from a nutritional and an organoleptic viewpoint. Most fruits are seasonal, many with quite short periods of availability. Distribution of various types of fruits depends to a large extent on latitude. Tropical fruits are not readily available in temperate and cold-climates, and temperate and cold climate fruits are not readily available in tropical areas.
The preservation of fruits represents a large industry. Common methods of preservation include canning, freezing, sun drying, freeze-drying and hot air drying.
The canned fruit industry is an extensive one and most fruits lend themselves to canning. The main disadvantages of canned fruits is their high cost of production, bulky nature, and associated high freight costs. Canned fruits are not very convenient for industrial use since can size is severely limited and cans are becoming increasingly more expensive.
Dried fruits, such as apple, pear, apricot, peach cherry etc are shelf-stable products, which, for storage at ambient temperatures, must be dried to moisture contents of 17% (or lower) to 23%. The drying of fruits to these low moisture levels requires long dehydration periods, often in the order of 16 to 24 hours. During this long period of dehydration, the sugars in the fruits become partially caramelized, the natural aroma of the fruit is lost and the color darkens. The retention of color during dehydration and storage can be maintained to some extent by the addition of sulfur dioxide or sulfites to the fruits, but this in itself adds a "sulfur" flavor, the use of sulfur dioxide is becoming unacceptable and the demand for additive-free fruit products is increasingly important.
Dried fruits, darken appreciably on storage, tend to be tough and leathery to eat and have flavors that differ substantially from that of fresh fruits.
Frozen fruits are common articles of commerce, but have not developed into a major consumer group of products mainly because frozen fruits are slow to thaw and, because of production of ice crystals in the fruit on freezing, there is substantial "drip" from the fruit on thawing. The tissues, as a result of the ice crystal damage, become mushy, and unattractive to eat unless consumed in a semi-frozen state.
A substantial market for fruits is in products such as yogurts and ice creams. Up to the present time, fruit of satisfactory quality for use in this type of produce is not readily available. Canned fruits tend to be too soft and "mushy" for use in yogurts, they are bulky to store and inconvenient to use since the cans have to be opened and the contents drained. They cannot be satisfactorily used in frozen desserts such as ice cream because the fruit pieces freeze hard and are therefore objectionable.
Frozen fruits are likewise unsuitable for ice creams because they are too hard. In yogurt they lack flavor intensity, have to be thawed for use with subsequent drip, are not sweet enough and are generally less than satisfactory.
Conventional dried fruits can be used in yogurt, but are unattractive in color and lack distinctive flavors. The presence of sulfur dioxide in the fruit can have a disadvantage for the reason that sulfur dioxide can be a deleterious effect on the yogurt organisms. If used in ice cream, the fruit pieces become excessively hard and lack natural flavor.
Some fruit products are known which are regarded as satisfactory for use in ice creams and similar products. The patents of Kahn, et al. (U.S. Pat. Nos. 4,332,824, 4,356,195, 4,418,082, 4,350,711, 4,390,550 and 4,551,384) describe the production of fruits and fruit products which are impregnated with sugar solution (where 50% of the sugars are in the form of dextrose or fructose). The fruits are produced by a form of osmotic dehydration, and are steeped in increasingly concentrated sugar solutions. These products will not freeze at normal freezer temperatures of 0.degree. C. to -15.degree. C., however, steeping causes loss of fruit flavor and acid and the products are excessively sweet. Moreover, the products tend to be expensive since considerable wastage of sugar solutions occurs.
Other fruit products have been described whereby partial dehydration of apples to about one-half their weight is followed by hard freezing as disclosed in U.S. Pat. No. of Jakobsson, et al. 4,647,469, U.S. Pat. No. 4,713,252, of Ismail, which coats fruit with sugar or sugar syrup to dehydrate and U.S. Pat. No. 3,408,208, of Lamb, which cooks and then partially dehydrates vegetables or meat and freezes the same; but in these cases, no attempt is made to produce a product which is resistant to becoming hard when frozen.
Lewis, et al. (Australian Patent No. 575,198) have described a process for the production of shelf-stable high moisture fruits. This process requires the use of low levels of sulfur dioxide in many fruits to avoid discoloration on storage and the products need to be held in an oxygen free atmosphere. There is now a universal consumer resistance to the use of even low levels of sulfur dioxide in all foods.